Intracellular Fibroblast Growth Factor (FGF) Homologous Factors (FHFs) interact with and regulate the activity of voltage-gated Na+ (Nav) channels in hippocampal neurons and in heterologous cells. A mutation (F145S) in FGF14/FHF4 is the cause of a progressive spinocerebellar ataxia and mental retardation syndrome in humans (SCA27), and mice lacking FGF14 (Fgf14-/-) have both motor and cognitive phenotypes that resemble the autosomal dominant FGF14F145S phenotype in humans. We hypothesize that FGF14F145S functions as a dominant negative protein in vivo by oligomerizing with wild type FGF14 or with other members of the FHF family to block their normal function. Although we have put substantial effort into understanding the function of FGF14 in vitro and when mis-expressed in hippocampal neurons or heterologous cells, further advances in understanding the physiological roles of FGF14 (and other intracellular FGFs) and defining the mechanisms underlying the phenotypic consequences of the SCA27 mutation in FGF14 are now limited by the lack of experimental tools that are necessary to probe these functions in vivo. To explore the function of FGF14F145S in vivo and to create a model for SCA27 with which to test potential therapeutic interventions for people with spinocerebellar ataxia, we will construct a mouse model in which the F145S mutation is inserted into the endogenous Fgf14 gene in mice and to generate monoclonal antibodies with which to detect and manipulate FHFs in vivo. Achieving these aims will allow detailed mechanistic studies into the physiological roles of the FHFs that are not presently possible. The following two specific aims are proposed: 1. To generate, characterize and validate monoclonal antibodies (Mabs) to FGF14 and other members of the intracellular FGF family. 2. To construct a mouse model of human spinocerebellar ataxia (SCA27) in which the F145S mutation is inserted into the endogenous Fgf14 gene using homologous recombination in embryonic stem cells.